1. Field of the Invention
The present invention relates to base station apparatuses and radio communication methods used in digital radio communication systems.
2. Description of the Related Art
A reception method using adaptive array antennas is known as the reception method by which signals received by a plurality of antenna elements are weighted/combined. The xe2x80x9cWaveform Equalization Technology for Digital Mobile Communicationsxe2x80x9d (compiled under the supervision of Jun Horigoshi, Triceps Co, Ltd.) describes that using an array antenna made up of a plurality of antennas to combine the antenna outputs after adding amplitudes and phase shifts changes array directivity. The adaptive array uses this principle. More specifically, it determines a weighting coefficient for each antenna output based on a certain control algorithm and controls directivity by adapting itself to changes in surrounding conditions.
FIG. 1 is a block diagram showing the configuration of the adaptive array reception apparatus above (hereinafter referred to as xe2x80x9creception adaptive arrayxe2x80x9d). As shown in FIG. 1, outputs 2 of a plurality of antennas 1 are multiplied by weighting coefficients 3, and these antenna outputs are combined by adder 7 into array output 4. The weighting coefficient corresponding to each antenna output is controlled by weighting coefficient control section 5. Weighting coefficient control section 5 controls weighting coefficients using three kinds of information, array output 4, antenna outputs 2 and preliminary knowledge 6 concerning a desired signal. There is also another method without using array output 4 for weighting coefficient control.
Conventionally, a weighting coefficient control algorithm assuming synchronous interference is used for control by weighting coefficient control section 5. According to this algorithm, it is only when an interference signal exists in the known signal portion of a desired wave that it is possible to determine such a weighting coefficient that will eliminate the interference signal.
However, in such a case where a desired signal transmitted by a communication user is mixed with an interference signal provided by another communication user, synchronization between the desired signal and interference signal is not guaranteed. This results, for example, in an interference signal mixing amidst the desired signal as shown in FIG. 4.
In FIG. 4, the interference signal starts to intrude amidst the desired signal at an interference mixing time, interfering with message section 302, but not with known signal section 301. Thus, when the interference signal intrudes amidst the desired signal, the reception adaptive array calculates weighting coefficients using known signal 301 free of the interference signal, and therefore it is not possible to remove the interference signal in the message section. This results in a problem that interference deteriorates the reception performance, increasing errors.
It is an objective of the present invention to provide a base station apparatus and radio communication method capable of improving the reception performance and reducing errors even if an interference signal intrudes amidst a desired signal.
This objective is achieved using a base station apparatus which adaptively controls a combination system according to the time (timing) at which the interference signal starts to mix with the desired signal.